In the present work, we explored the interplay between the lattice and magnetic degrees of freedom in a rare-earth nickelate ${\text{EuNiO}}_3$, by carrying out temperature-dependent structural analysis in conjunction with distortion mode analysis. The temperature-dependent powder synchrotron x-ray diffraction (SXRD) studies revealed the presence of an orthorhombic $Pbnm$ phase (tilt system $a_0^{-}{a}_0^{-}{c}_0^{+}$), with an elementary perovskite (pseudomonoclinic) cell, over the analyzed temperature range, i.e., 100–623 K. Further, we observed two distinct anomalies in the temperature-dependent evolution of pseudomonoclinic cell parameters $(c_p/a_p,$ $\gamma ,$ $V_{\text{mono}})$ around 463 K and 200 K corresponding to respective isosymmetric metal-insulator transition temperature $\left(T_{M\text{−}I}\right)$, and Neel temperature $\left(T_N\right)$ linked with a volume gain at low temperatures dictating a magnetoelastic coupling in the system. We show the existence of two distinct pseudomonoclinic phases, viz., ${\mathrm{Mono}}_{\mathrm{metal}}$ (T > $T_{M\text{−}I}$) and ${\mathrm{Mono}}_{\mathrm{insulator}}$ (T < $T_{M\text{−}I}$), where the latter is more distorted than the former. The transition from ${\mathrm{Mono}}_{\mathrm{metal}}$ to ${\mathrm{Mono}}_{\mathrm{insulator}}$ at $T_{M\text{−}I}$ is reminiscent of a phase transition from orthorhombic (metallic) to monoclinic (insulating) phase, observed in other members of the rare-earth family. In addition, $T_{M\text{−}I}$ and $T_N$ are clearly evident by the nonanalytical behavior of the condensed soft phonon modes amplitude corresponding to the zone boundary of the cubic Brillouin zone, viz., $X_5^{+}(q=0,1/2,0)$ and $R_5^{+}(q=1/2,1/2,1/2)$ as a function of temperature.

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• Crystal phenomena
• Magnetic interactions
• Magnetic order
• Metal-insulator transition
• Specific phase transitions
• Perovskites